Question: On the first 5 tests of his math class, Christopher got an average score of 87. What does he need on the next test to have an overall average of 89?
Answer: Let his score on the next test be $x$ The sum of all of his scores is then $5 \cdot 87 + x$ The same sum must also be equal to $6 \cdot 89$ Solve: $x = 6 \cdot 89 - 5 \cdot 87 = 99$.